This invention relates to pressure sensors and more particularly to a pressure sensor employing a semiconductor convoluted diaphragm.
Typical prior art pressure sensors employing piezoresistive devices operated with a thin "coin-like" diaphragm of a circular, square or rectangular plan configuration. The piezoresistive sensors were then diffused or otherwise deposited upon the diaphragm.
Many examples of such structures exist in the prior art. See for example U.S. Pat. No. 3,654,579 entitled ELECTROMECHANICAL TRANSDUCERS AND HOUSINGS by A. D. Kurtz et al issued on Apr. 14, 1972 and assigned to the assignee herein.
In displacement or pressure transducers the diaphragm is subjected to a force the magnitude of which is designated by the resistance change of the sensors. The desirable mode of operation for such diaphrams is bending. This bending produces surface fiber strains which are sensed by piezoresistive elements. In any event, at low pressure and large forces the diaphragm apart from bending tends to stretch. The stretching of the diaphragm is a nonlinear function of applied force or pressure.
A diaphragm may be looked upon as a spring element which supports the applied laod. Deflection of the center is caused by the application of load. If all the applied load is supported by bending, then this deflection is linear. If on the other hand, a portion of the applied load is resisted by a restoring force caused by stretching, then little deflection is caused by this portion of the load. It is found that the center deflection becomes nonlinear in a regressive and in turn the fiber strains are similarly nonlinear. This stretch effect is very noticeable and objectionable in low pressure transducer structures because such structures employ thin diaphrams which off little resistance to stretching. In general such low pressure piezoresistive transducers must be down rated or reduced in full range sensitivity to maintain linear response at low pressure. The greater the stiffness of the diaphragm, the greater the pressure that the diaphragm can handle and thick diaphragms are usable as high pressure transducers. Thin diaphragms are rather difficult to manufacture and therefore low pressure diaphragms require a relatively large area.
In order to alleviate the nonlinearity due to stretching, the prior art employed convoluted or corrugated diaphragms. These diaphragms exhibit a large range of deflection to applied pressure and therefore the stretching effect was not as pronounced. Typically, such diaphragms are used in conjunction with a push rod and beam to form a relatively complex pressure responsive structure. In any event, such diaphragms were machined from a suitable metal and were and are extremely difficult to manufacture. Furthermore, the strain sensors or gages were separately positioned and mounted on the convoluted structures or associated ancillary flexures resulting in additional problems which affected the transducer performance and linearity. These problems were inherent in the bonding techniques used to secure the sensors to the diaphragm, the dissimilar materials and associated temperature effects and a host of other problems.
It is therefore desirable to provide a convoluted silicon diaphragm for low pressure operation (15 psi or less) wherein semiconductor sensors are directly diffused into the diaphragm by integrated circuit techniques. Semiconductor technology provides a precise, economical and effective means of both fabricating the complex nonplanar diaphragm structure and integrating the piezoresistive elements.